The Union Carbide Corporation (UCC) researched and developed a series of novel silicoaluminophosphate molecular sieves SAPO-n in 1984 (U.S. Pat. No. 4,440,871 and U.S. Pat. No. 4,499,327), which were prepared by using organic amines as a template agent and using hydrated alumina, phosphoric acid and silica sol as an aluminum source, a silicon source and a phosphorus source, respectively. Firstly, a complex of organic amine/silicoaluminophosphate with microporous structure was obtained by a hydrothermal crystallization method, and then the template agent (organic amine) was removed by calcination to obtain the SAPO-n molecular sieves. Among this kind of molecular sieves, SAPO-34 with CHA-type framework has shown an excellent catalytic activity and selectivity in methanol to olefins (MTO) process, due to its proper pore structure, proper Brønsted acid property, higher specific surface area, preferable adsorption performance, high thermal stability, high hydrothermal stability and the like.
SAPO-34 is a molecular sieve with chabazite-type (CHA) framework containing 8-member ring ellipsoidal cage and 3-dimensional channel, which is formed by stacking of double six-rings according to ABC sequence. SAPO-34 is microporous molecular sieve with a pore size of 0.38×0.38 nm. Space group of SAPO-34 is R3m belonging to trigonal crystal system (J. Phys. Chem., 1990, 94: 2730). SAPO-34 is formed by Si, Al, P and O whose composition change at some range, generally in the order of n(Si)<n(P)<n(Al).
SAPO-34 molecular sieve is generally produced by a hydrothermal synthesis process which uses water as the solvent and is conducted in a sealed autoclave. The silicon source may be chosen from silica sol, active silica and orthosilicate esters. The aluminum source may be chosen from active alumina, pseudo boehmite and alkoxy aluminum. Preferable silicon source and aluminum source are silica sol and pseudo boehmite. Phosphorus source is generally 85% phosphoric acid. The template agent commonly used comprises tetraethyl ammonium hydroxide (TEAOH), morpholine (MOR), piperidine, isopropylamine triethylamine diethylamine (TEA), dipropylamine (DEA), and the like, or a mixture thereof. The structural-directing agent partly affects the microstructure, elemental composition, morphology of synthesized molecular sieve, thus producing an impact on the catalytic performance of synthesized molecular sieve.
In the present invention, diglycolamine firstly is used as structure-directing agent for the hydrothermal synthesis of pure SAPO-34 molecular sieve. The synthetic SAPO-34 molecular sieve has shown an excellent catalytic performance in catalytic reaction and a good gas absorption separation performance.